1Earth and Life in theLate Archaean-Early ProterozoicThe major early events:1. Oxygenic (aerobic) photosynthesis.Pioneer organism was probably cyanobacteria(Only anaerobic photosynthesis previously, plus other dominantmetabolisms, especially methanogens. Atmosphere probablymethane-rich until this point potent greenhouse gas).Why important?…2. Great Oxidation EventMay have triggered climate disaster (Snowball Earth) and manyof the subsequent biological innovations, including multicellularmicroorganisms, eukaryotic cells, many others.The major questions have to do with the relation between these two.2The Late Archean/Early Proterozoic Eon3Atmospheric oxygen:Master of evolution since ~ 2 Gyr ago?Evidence: 1. Banded iron formations: Form from minerals dissolved in ocean, but thisonly happens in absence of significant oxygen. So oxygen very low ~ 0.01% back to ~ 2-3 Gyr ago.2. Sulfur isotope ratios: SO2, H2O emitted by volcanoes, but sulfur isotoperatios are altered in presence of oxygen. Shows oxygen very low~ 0.002% before 2.35 Gyr. 3. “Great oxygen event” at ~ 2.1 Gyr ago. But unsure how much oxygenincreased or how long it took (textbook mentions it may have remained fairly low, ~ 1%, until Cambrian ~ 0.5 Gyr ago).4Evolution of the composition of Earth’s atmosphere(this is a simplification--see next slide)5Great Oxidation Event(s)After a minor increase at ~ 3.2 Gyr, a jump by a factor of ~ 100 occurred around2.2 Gyr ago. Subsequent wiggles may also be important for terrestrial biology.Note logarithmicscale6Aerobic photosynthesis and the rise of oxygenEarly life was anaerobic, and developed glycolysis for energy transfer, a pathway stillused in all known cells. Glycolysis was sufficient for single-cell life, but no knownexamples of multicellular complex organisms that are exclusively anaerobic. Furthercomplexity apparently awaited pathways for larger energy transfer: aerobicphotosynthesis. For example thiamin and B12 synthesis are old anoxic pathwaysrewired to take advantage of oxygen.When did oxygenic photosynthesis evolve? The question is significant because photosynthetic oxygen production by cyanobacteria led to oxygenation of the atmosphere and oceans, in turn allowing aerobic respiration and the evolution of large, complex and ultimately intelligent organisms.7Summary of possible evidence related to timing ofoxygenic photosynthesis and GOE??XXEarliest convincing eukaryotic fossils?? “GOE”“Cambrian explosion”2008: Earliest convincingevidence for cyanobacteriaZircons: Oceans, continents, atmosphere in placeClement conditions punctuated by sterilizing impacts8A timing problem (now vanished?)Because cyanobacterial lipid biomarker and other evidence was somuch earlier than the Great Oxidation Event, needed to store O2 incrust until saturated, then released from vents, volcanoes.Kump et al. (2005): Emergence of volcanoes from the sea9See Buick 2008 abstract below: Argues that there isstrong evidence for cyanobacteria at very early timesAbstract: The atmosphere has apparently been oxygenated since the 'Great Oxidation Event' ca 2.4 Ga ago, but when thephotosynthetic oxygen production began is debatable. However, geological and geochemical evidence from oldersedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils inca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preservedwithout subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Reabundances and sulphur isotope systematics of slightly older ( 2.5 Ga) kerogenous shales record a transient pulse ofatmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient inexogenous reducing power strongly imply that oxygen producing cyanobacteria had already evolved. Even at ca 3.2 Ga,thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U-Pb data fromca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, thehypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems wellsupported.Buick, R. 2008 PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICALSCIENCES Volume: 363 Issue: 1504 Pages: 2731-2743See meeting issue “Photosynthetic and atmospheric evolution”Philosophical Transactions of the Royal Society B 363, 15 04 Aug 2008, Can access through UTNetLibrary10Eukaryotes and EndosymbiosisCytoskeletonEukaryotic (animal) cell: Hugeincrease in structural complexity.Organelles former free-livingprokaryotes?Prokaryotic cellEukaryotic microorganismsEukaryotic cellSuggestion: Look up“endosymbiosis”11A few suggested results of the Great Oxygenation Event1. Destroyed the methane greenhouse from methanogenic bacteria,triggering a snowball event on a short timescale.3. Greatly enlarged range of metabolic networks. Beforeoxidation event, only glycolysis. Larger organisms than single cellsrequired more efficient energy producers.New metabolic networks enlarged pathways for synthesis of metabolites.Thiamin and B12 synthesis are two well-known examples.2. Ultra-high reactivity of oxygen resulted in extinction of most organisms, who were unprepared to adapt. Some anaerobes did survive,but the world of larger organisms was to be aerobic from this time on.There are no known multicellular organisms that are strictly anaerobic.Many more--ask me if you want references. For example:Evolution, atmospheric oxygen, and complex disease.L. G. Koch and S. L. Britton (2007) Physiol Genomics 30, 205-20812Escalating development of complexity after the Great Oxidation EventThe main uncertainty: Whether oxygenic photosynthesis occurred more than 0.1Gyr before theGOE (as much as 1 Gyr according to some), and if so, how was oxygen stored in crust for so long? Not shown is Proterozoic Snowball Earth episode, which may come just after GOE Note transition from single to multicellular organisms shortly after GOE. Eukaryotes earlier than 1.5 Gyr. Next large increase in oxygen at 0.5-1.0 Gyr preceeded Cambrian explosion during which allanimal phla appeared.13Oxygen’s imprint on genome complexity and metabolic pathwaysSpeculative overviewTracing Oxygen's Imprint on Earth'sMetabolic Evolution. Paul G. FalkowskiScience 24 March 2006: Vol.
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